Image display device and evaluating method of glass substrate for use in it

ABSTRACT

The present invention provides an image display device capable of displaying a good image by suppressing yellowing of a glass substrate, and an evaluating method of the glass substrate. The image display device is formed using the glass substrate where reflectance at wavelength of 220 nm is 5% or lower. In the evaluating method of the glass substrate for the image display device, Sn ++  content in the glass substrate is analyzed using reflectance at wavelength of 220 nm.

This application is a divisional of application Ser. No. 10/499,339,which is the National Stage of International Application No.PCT/JP2003/015123, filed Nov. 27, 2003.

TECHNICAL FIELD

The present invention relates to an image display device such as aplasma display panel (PDP) and an evaluating method of a glass substratefor use in it.

BACKGROUND ART

There are various types of image display devices for displaying a highdefinition television image on a large screen. PDPs belong to one of thevarious types. A PDP is hereinafter described as an example.

A PDP is formed of two glass substrates: a front-side glass substratefor displaying an image; and a back-side glass substrate facing thefront-side glass substrate. The front-side glass substrate has thefollowing elements:

-   -   a display electrode that is formed on one principal surface        thereof and includes a stripe-like transparent electrode and a        bus electrode;    -   a dielectric film that covers the display electrode and works as        a capacitor; and    -   a MgO protective layer formed on the dielectric film.        While, the back-side glass substrate has the following elements:    -   a stripe-like address electrode formed on one principal surface        thereof;    -   a dielectric film for covering the address electrode;    -   barrier ribs formed on the dielectric film; and    -   phosphor layers that are formed between the barrier ribs and        emit red light, green light, and blue light, respectively.

As the front-side glass substrate and the back-side glass substrate,glass substrates that are easily increased in area, have high flatness,are inexpensive, and are manufactured by a float method are used. Theseglass substrates are disclosed in Electronic Journal, Separate Volume“2001, FPD Technology Summa” (Electronic Journal Co. Ltd. Oct. 25, 2000,p706-p707).

The float method is a method of forming plate-shaped glass by floatingand conveying molten glass onto molten metallic tin under reducingatmosphere. An inexpensive glass sheet having large area can beprecisely manufactured in the float method, so that the float method isin widespread use in manufacturing of a window glass or the like.

When an Ag electrode made of silver material is formed on a float glasssubstrate manufactured by the float method, however, a colored layer isdisadvantageously formed on the surface of the glass substrate and theglass substrate changes into yellow (yellows).

This coloring phenomenon of the glass substrate by the Ag electrode iscaused by the following processes:

-   -   a silver colloid is generated by oxidation-reduction reaction        between reducing bivalent tin ions (Sn⁺⁺) existing on the glass        substrate and silver ions (Ag⁺); and    -   light absorption therefore occurs near wavelength of 350 to 450        nm.

In other words, the glass substrate is exposed to the reducingatmosphere containing hydrogen in a molding process in a float furnaceas a molten metallic tin bath. A reducing layer with a thickness ofseveral μm containing tin ions (Sn⁺⁺) of the molten tin (Sn) isgenerated on the surface of the glass substrate. When a bus electrodeincluding an Ag electrode is formed on the glass substrate having thereducing layer on its surface, silver ions (Ag⁺) separate from the buselectrode, and infiltrate into the glass due to ion exchange with alkalimetal ions contained in the glass. The infiltrating silver ions (Ag⁺)are reduced by the tin ions (Sn⁺⁺) existing in the reducing layer togenerate metallic silver (Ag) colloid. The metallic silver (Ag) colloidyellows the glass substrate. The yellowing occurs also on the front-sideglass substrate having the bus electrode on the transparent electrode.

When the glass substrate, especially the front-side glass substrate,yellows, the yellowing is fatal in the image display device. Due to theyellowing of the glass substrate, the panel looks yellow, the commercialvalue decreases, display brightness of blue decreases to change displaychromaticity, and, color temperature decreases to degrade picturequality especially in displaying white.

These problems occur not only in a PDP but also in a general imagedisplay device having a structure where an Ag electrode is formed on aglass substrate.

The present invention addresses the problems, provides an image displaydevice allowing good image display by suppressing yellowing of the glasssubstrate, and provides an evaluating method of the glass substrate foruse in the image display device.

DISCLOSURE OF THE INVENTION

An image display device of the present invention, for addressing theproblems, employs a glass substrate of which reflectance at a wavelengthof 220 nm is 5% or lower.

Thanks to this structure, even in the image display device where anelectrode made of Ag material is formed on the glass substratemanufactured by the float method, the glass substrate does not yellowand the image display quality is high.

In the evaluating method of the glass substrate for the image displaydevice of the present invention, content of Sn⁺⁺ in the glass substrateis analyzed based on the reflectance at wavelength of 220 nm. Inproviding an image display device by forming an electrode made of Agmaterial on a glass substrate manufactured by the float method, thisevaluating method allows easy and efficient selection of a glasssubstrate that does not yellow and provision of a glass substrateoptimum for an image display device having high image display quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional and perspective view showing a schematic structureof a PDP as an image display device in accordance with an exemplaryembodiment of the present invention.

FIG. 2 is a graph showing a relation between a surface removing amountand a reflection spectrum of a glass substrate manufactured by a floatmethod.

FIG. 3 is a graph showing a relation between reflectance at thewavelength of 220 nm and coloring degree of glass.

FIG. 4 is a graph showing difference ΔR between reflection spectrumR_(S)(λ) of the glass substrate and reflection spectrum R_(B)(λ) in anonexistent state of Sn⁺⁺.

FIG. 5 is a graph showing an analyzing result of the reflection spectrumof the glass substrate.

FIG. 6 is a graph illustrating wavelength λ* maximizing difference ΔRbetween reflection spectrum R_(S)(λ) of the glass substrate andreflection spectrum R_(B)(λ) in a nonexistent state of Sn⁺⁺.

FIG. 7 is a schematic block diagram of a manufacturing apparatus of theglass substrate for the image display device in accordance with theexemplary embodiment of the present invention.

FIG. 8 is a schematic block diagram of another manufacturing apparatusof the glass substrate for the image display device in accordance withthe exemplary embodiment.

FIG. 9 is a schematic block diagram of still another manufacturingapparatus of the glass substrate for the image display device inaccordance with the exemplary embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

An exemplary embodiment of the present invention will be described withreference to the drawings.

A PDP is hereinafter described as an example of image display devices.However, the present invention is useful for an image display devicehaving a structure where an electrode made of Ag material is disposed ona glass substrate that is manufactured by the float method and has Sn⁺⁺on its surface.

FIG. 1 is a sectional and perspective view showing a schematic structureof the PDP. PDP 1 is formed of two glass substrates: front-side glasssubstrate 3 for displaying an image; and back-side glass substrate 10facing the front-side glass substrate.

Front substrate 2 of PDP 1 has the following elements:

-   -   display electrodes 6 that are formed on one principal surface of        front-side glass substrate 3 and include scan electrode 4 and        sustain electrode 5;    -   dielectric layer 7 for covering display electrodes 6; and    -   protective layer 8, made of MgO for example, for covering        dielectric layer 7.        In scan electrode 4 and sustain electrode 5, for decreasing        electric resistance, bus electrodes 4 b and 5 b made of Ag        material are laminated on transparent electrodes 4 a and 5 a,        respectively.

Back substrate 9 has the following elements:

-   -   address electrodes 11, made of Ag material, formed on one        principal surface of back-side glass substrate 10;    -   dielectric layer 12 for covering address electrodes 11;    -   barrier ribs 13 formed on dielectric layer 12 at positions        corresponding to clearances between address electrodes 11; and    -   phosphor layers 14R, 14G, and 14B between barrier ribs 13.

Front substrate 2 faces back substrate 9 with barrier ribs 13 sandwichedso that display electrodes 6 are orthogonal to address electrodes 11,and the outer periphery of the image display region is sealed by asealing member. Discharge spaces 15 formed between front substrate 2 andback substrate 9 are filled with discharge gas such as Ne—Xe 5% atpressure of 66.5 kPa (500 Torr).

Crossing parts between display electrodes 6 and address electrodes 11 indischarge spaces 15 work as discharge cells 16 (unit light emittingregions).

As front-side glass substrate 3 and back-side glass substrate 10, glasssubstrates that are easily increased in area, have high flatness, areinexpensive, and are manufactured by a float method are used.

In the structure discussed above, bus electrodes 4 b and 5 b onfront-side glass substrate 3 are formed of Ag electrodes. If front-sideglass substrate 3 contains Sn⁺⁺, the glass substrate yellows even wheneach of transparent electrodes 4 a and 5 a is interposed between each ofbus electrodes 4 b and 5 b and glass substrate 3. Depending on thedegree of the yellowing, an image display characteristic of the imagedisplay device is adversely affected.

The glass substrate used as front-side glass substrate 3 of PDP 1 isanalyzed to determine Sn⁺⁺ content on the surface thereof on which buselectrodes 4 b and 5 b containing Ag are to be formed. When theappearance quality is concerned, back-side glass substrate 10 is alsoanalyzed to determine Sn⁺⁺ content thereof on the surface on whichaddress electrodes 11 containing Ag are to be formed.

Specifically, reflectance of the glass substrate at the wavelength of220 nm is measured, and the analysis is performed based on thereflectance. This method is provided based on inventors' study. Theinventors found the following facts:

-   -   the reflectance near the wavelength of 220 nm increases with        increase in Sn⁺⁺ content on the glass substrate; and    -   there is a correlation between the reflectance near the        wavelength of 220 nm and coloring of the glass substrate by        silver colloid.        Here, the reflectance may be measured by a general measuring        device.

The Sn⁺⁺ content on the glass substrate is determined by a secondaryion-mass spectrometry (SIMS) or an inductively-coupled plasma (ICP)optical emission spectrometry. An allowance of Sn⁺⁺ content isdetermined based on a calibration curve derived from the relationbetween the Sn⁺⁺ content determined by the spectrometry and the measuredreflectance. The allowance of Sn⁺⁺ content can be therefore determinedfrom the reflectance without breaking the glass substrate.

In other words, firstly, the surface on the non-contact side with tin(top surface) of the glass substrate manufactured by the float method isuniformly removed by thickness of 3, 7, 15, or 20 μm, and reflectionspectrum of the remaining glass substrate is measured at wavelength of200 to 300 nm. The measurement result is shown in FIG. 2. FIG. 2 alsoshows a measurement result of a glass substrate without removal forcomparison. The reason why the top side surface is removed is asfollows. Adhesion and diffusion amounts of tin are less and hence theyellowing degree is lower on the top-side surface than the bottom-sidesurface (contact side with tin), so that a bus electrode made of Agmaterial is generally formed on the top-side surface. When the Agelectrode is formed on the bottom side, the coloring degree is two orthree times higher than that in the case that the Ag electrode is formedon the top side.

FIG. 2 shows that, when removed thickness is 15 μm or less, reflectanceat peak A near the wavelength of 220 nm decreases with increase of theremoved thickness. When removed thickness is 15 μm or more, the decreaseof the reflectance stops. It is considered that Sn⁺⁺ contentmonotonously decreases in the depth direction from the top side of theglass substrate. The result shown in FIG. 2 matches with theconsideration, and hence the decrease of the reflectance at peak A isconsidered to match with the decrease of Sn⁺⁺ content.

Next, for clarifying a relation between the peak near the wavelength of220 nm appearing in the reflection spectrum and yellowing of the glasssubstrate, an Ag electrode is formed on the glass substrate and coloringdegree of the glass substrate is measured. In other words, 5 μm thicksilver paste as the Ag electrode is applied onto the glass substrate byscreen printing, they are calcined at 600° C., and a relation betweenthe coloring degree of the glass substrate and reflectance at thewavelength of 220 nm is investigated. FIG. 3 shows the investigationresult. The coloring degree of the glass substrate is evaluated using b*in a L*a*b* color system (JIS Z 8729). The larger b* value is, thehigher the degree of yellowing is. The coloring degree of the glasssubstrate is measured from the side having no Ag electrode. As shown inFIG. 3, the reflectance of light at the wavelength of 220 nm on theglass substrate and the coloring degree b* of the glass substrate aredirectly proportional.

The investigation result discussed above shows that increase of thereflectance of the glass substrate at the wavelength of 220 nm has acorrelation to the Sn⁺⁺ content in the glass substrate, namely contentof reducing material at least causing yellowing. Therefore, by measuringthe reflectance at the wavelength of 220 nm, the Sn⁺⁺ content in theglass substrate on which the Ag electrode is to be formed can beanalyzed based on the calibration curve, and the degree of yellowing ofthe glass substrate can be also estimated. This method is useful forevaluating whether or not a selected glass substrate is optimum for animage display device.

In FIG. 2, after 15 μm or more thick glass substrate is removed, thereflectance (about 2%) near the wavelength of 220 nm is not resultedfrom existence of Sn⁺⁺ but by bottom part of a reflection spectrumhaving a peak at another wavelength. The stop of the decrease of thereflectance at the wavelength of 220 nm is considered to be caused bydecrease of the Sn⁺⁺ content in the glass substrate. FIG. 4 showsdifference ΔR(λ)=R_(S)(λ)−R_(B)(λ). Here, R_(S)(λ) is a reflectionspectrum of the glass substrate shown in FIG. 2, and R_(B)(λ) is areflection spectrum in a nonexistent state of Sn⁺⁺, namely in a statewhere the removal of 15 μm or more thick glass substrate stops thedecrease of the reflectance. Reflection difference ΔR is considered toindicate the existence of Sn⁺⁺.

The reflectance at the wavelength of 220 nm may be read from areflection spectrum distribution as shown in FIG. 2. However, for moreprecisely evaluating signal strength of the reflection spectrum having acorrelation to Sn⁺⁺, the following method can be used. Reflectionspectrum is measured in a wider range of wavelength, for example 180 to280 nm. The measured reflection spectrum is then divided into twoGaussian peaks of a component having a correlation to Sn⁺⁺ and acomponent having no correlation to Sn⁺⁺ shown in FIG. 5 by a curvefitting method using $\begin{matrix}{{M\quad 1\exp\left\{ {- \frac{\left( {{1240/\lambda}\quad - {{1240/M}\quad 2}} \right)^{2}}{M\quad 3^{2}}} \right)} + {M\quad 4\exp{\left\{ {- \frac{\left( {{1240/\lambda} - {{1240/M}\quad 5}} \right)^{2}}{M\quad 6^{2}}} \right\}.}}} & \left( {{Eq}.\quad 1} \right)\end{matrix}$Where, λ is a wavelength (nm), and M1 to M6 are fitting parameters.

The lower limit of the measured wavelength range is set at 180 nmbecause oxygen in atmospheric air absorbs light at a wavelength lowerthan 180 nm, hence vacuum or atmosphere containing no oxygen is requiredfor measurement, and construction of a measuring system and measurementrequire much effort.

This method is also useful for evaluating whether or not a selectedglass substrate is optimum for an image display device.

The position of the wavelength of the peak of the reflectance caused bySn⁺⁺ can slightly change depending on the manufacturing condition andthe composition of the glass substrate. Therefore, for increasinganalysis accuracy of Sn⁺⁺, it is more effective to analyze not only thereflectance at wavelength of 220 nm but also the bottom part of thereflectance extending to wider wavelength range, for example 200 to 250nm.

Specifically, wavelength λ* maximizing differenceΔR(λ)=R_(S)(λ)−R_(B)(λ) in a wavelength range of 200 to 250 nm isconsidered to indicate the existence of Sn⁺⁺, as shown in FIG. 6. Here,R_(S)(λ) is a reflection spectrum of the glass substrate, and R_(B)(λ)is a reflection spectrum in a nonexistent state of Sn⁺⁺. In FIG. 6,wavelength λ* maximizing ΔR(λ) is additionally written in FIG. 4. Sn⁺⁺content in the glass substrate is analyzed based on the reflectanceR_(S)(λ*) at wavelength λ* or reflectance differenceΔR(λ*)=R_(S)(λ*)−R_(B)(λ*).

The reflectance difference ΔR(λ*)=R_(S)(λ*)−R_(B)(λ*) means the maximumvalue of ΔR(λ)=R_(S)(λ)−R_(B)(λ). Here, R_(S)(λ) is a reflectionspectrum of the glass substrate at wavelength of 200 to 250 nm, andR_(B)(λ) is a reflection spectrum in a nonexistent state of Sn⁺⁺.

Sn⁺⁺ locally exists only in a region from the outermost surface of theglass substrate to depth of about 15 μm, as shown in FIG. 2. Therefore,reflection spectrum on the glass substrate of which top part havingthickness of 15 μm or more, preferably 20 μm or more, is removed can beset as reflection spectrum R_(B)(λ) in the nonexistent state of Sn⁺⁺.

Another specific method of analyzing reflection spectrum also includingthe extending bottom part of the reflection spectrum is provided asfollows. A mean reflectance is determined from area integral ofreflection spectrum at the wavelength of 200 to 250 nm, for example, andSn⁺⁺ content is analyzed.

Either of the methods discussed above is useful for evaluating whetheror not a selected glass substrate is optimum for an image displaydevice.

A judgment standard for the analysis result of the Sn⁺⁺ content on thesurface of the glass substrate on which the Ag electrode is to be formedis described hereinafter.

Existence of Sn⁺⁺ reduces Ag⁺ of the Ag electrode to generate Agcolloid, and the glass substrate yellows. The coloring (yellowing)degree of the glass substrate is determined based on the Sn⁺⁺ content,so that an allowance of the Sn⁺⁺ content is a judgment standard when theglass substrate is used for an image display device.

As shown in the result of FIG. 2, for preventing the yellowing, it ispreferable that the following parameter is smaller:

-   -   reflectance at a wavelength indicating existence of Sn⁺⁺, such        as reflectance R_(S)(220) at the wavelength of 220 nm;    -   reflectance R_(S)(λ*) at wavelength λ* maximizing reflection        spectrum difference ΔR(λ)=R_(S)(λ)−R_(B)(λ);    -   reflectance difference ΔR(λ*)=R_(S)(λ*)−R_(B)(λ*); or    -   mean reflectance R_(S-mean) (200-250) at wavelength of 200 to        250 nm. Specifically, reflectance R_(S)(220) is 5% or less,        reflectance R_(S)(λ*) is 5% or less, reflectance difference        ΔR(λ*) is 3% or less, or mean reflectance R_(S)-mean (200-250)        is 5% or less. In this case, it is verified that the Sn⁺⁺        content is so low that the yellowing of the glass substrate        presents no problem even when an image display device is        manufactured by forming an Ag electrode on the glass substrate.

However, the low Sn⁺⁺ content in the glass substrate can be caused byweak reducing force of the atmosphere in a float furnace. In this case,disadvantageously, metallic tin in a tin bath continuously oxidizes andvolatilizes in manufacturing the glass substrate. Too low Sn⁺⁺ contentin the glass substrate is not preferable in manufacturing the glasssubstrate.

It is therefore preferable that reflectance R_(S)(220) is between 2.5%and 5%, reflectance R_(S)(λ*) is between 2.5% and 5%, reflectancedifference ΔR(λ*) is between 0.5% and 3%, or mean reflectance R_(S)-mean(200-250) is between 2.5% and 5%.

When a measured reflectance of the glass substrate exceeds the rangediscussed above, Sn⁺⁺ content exceeds an allowance where yellowing ofthe glass substrate is prevented from affecting the image display. Inthis case, when an image display device is manufactured by forming an Agelectrode on the glass substrate, yellowing producing a defect in theimage display device occurs.

When the Sn⁺⁺ content is determined to exceed the allowance, thereducing force in a float furnace is weakened in manufacturing the glasssubstrate, and the Sn⁺⁺ content of the glass substrate is decreased. Forweakening the reducing force in the float furnace, specially, hydrogenconcentration in the float furnace is deceased. Mixed gas of hydrogenand nitrogen is generally used as atmospheric gas in the float furnace.The mixed gas contains 2 to 10 vol % of hydrogen. The reducing force iscontrolled by changing hydrogen concentration in this hydrogenconcentration range in response to the allowance of the Sn⁺⁺ content.

FIG. 7 shows a manufacturing apparatus of the glass substrate. Amanufacturing method of the glass substrate is described.

Material for the glass substrate is injected into melting furnace 21,heated to a high temperature to be molten, and then supplied to floatfurnace 22. Float furnace 22 has molten tin 24 in its lower part, andhas reducing atmosphere 25 (mixed gas of hydrogen and nitrogen) forpreventing oxidation of tin in its upper space. Molten glass iscontinuously moved on molten tin 24 and molded as plate-like glassribbon 23. Glass ribbon 23 is lifted up from the tin bath and moved toslow cooling furnace 27 by conveying roller 26. Distortion occurringduring the molding is decreased by slowly cooling glass ribbon 23 inslow cooling furnace 27.

After the slow cooling process, a surface analyzing process of measuringreflectance with reflectance measuring device 32 and analyzing the Sn⁺⁺content of the glass substrate is performed in the manufacturingapparatus in FIG. 7. Reflectance measuring device 32 measures thefollowing parameter:

-   -   reflectance at a wavelength indicating existence of Sn⁺⁺ in the        glass substrate, such as reflectance R_(S)(220) at the        wavelength of 220 nm;    -   reflectance R_(S)(λ*) at wavelength λ* maximizing        ΔR(λ)=R_(S)(λ)−R_(B)(λ);    -   reflectance difference ΔR(λ*)=R_(S)(λ*)−R_(B)(λ*); or    -   mean reflectance R_(S-mean) (200-250) at wavelength of 200 to        250 nm.

When the Sn⁺⁺ content is determined to exceed the allowance based on themeasured reflectance, concentration of the atmosphere gas is controlledto weaken the reducing force in float furnace 22. For preventingyellowing, it is preferable that the reflectance is as low as possible.While, when the reducing force of atmosphere 25 in float furnace 22 isexcessively weakened for reducing the Sn⁺⁺ content in the glasssubstrate, disadvantageously, metallic tin contained in molten tin 24continuously oxidizes and volatilizes in manufacturing the glasssubstrate.

Therefore, when the reflectance corresponding to the Sn⁺⁺ content in theglass substrate is higher than the allowance value discussed above, thehydrogen concentration of the atmosphere in the float furnace iscontrolled to be decreased. When the reflectance is lower than theallowance value, the hydrogen concentration is preferably increased forpreventing oxidation of the metallic tin.

This reflectance measurement can be performed nondestructively, in anon-contact matter, and in a short time, so that the measurement isapplicable also to a process control of a routine manufacturing processof a glass substrate. The image display device is especially required tobe uniform on its surface, so that the reflectance is preferablymeasured at a plurality of positions for recognizing dispersion on theglass substrate.

The Sn⁺⁺ content can be evaluated by the secondary ion-mass spectrometry(SIMS) or the inductively-coupled plasma (ICP) optical emissionspectrometry. However, these methods are destructive inspections and canhardly used for measurement on a large area, so that the methods areinappropriate for in-line measurement of Sn⁺⁺ content in a glasssubstrate in the glass substrate manufacturing process. When Sn⁺⁺content in a predetermined sample is measured, reflectance of the sampleis measured, and a calibration curve is prepared, however, Sn⁺⁺ contentcan be determined based on the reflectance.

When hydrogen concentration of the atmosphere in the float furnaceincreases, reducing property of the atmosphere is increased to increasethe Sn⁺⁺ content of the glass substrate, and the yellowing of the glasssubstrate presents a problem, as discussed above. The variation in theSn⁺⁺ content of the glass substrate appears as difference in yellowingdegree of the glass substrate, so that this variation must be within acertain range. When the reflectance of the glass substrate is higherthan the predetermined range discussed above, the hydrogen concentrationin the float furnace is decreased. The decreasing weakens the reducingproperty of the atmosphere, so that the reflectance of the glasssubstrate can be decreased.

After the surface analyzing process of measuring reflectance, in acutting process, glass ribbon 23 is cut into an arbitrary size by acutter 28 and glass substrate 100 is produced.

Though the reducing force in float furnace 22 is controlled to weaken,the analyzed Sn⁺⁺ content of the glass substrate on which an Agelectrode is to be formed sometimes exceeds the allowance. In this case,as shown in FIG. 8, the Ag electrode receiving surface of the glasssubstrate is partially removed until the Sn⁺⁺ content becomes within theallowance in surface removing furnace 29 in a surface removing process.In other words, by controlling the reducing force in float furnace 22 toweaken and partially removing the surface of the glass substrate, thesurface of the glass substrate formed so as to have decreased Sn⁺⁺content is further partially removed. In this case, removed thicknesscan be decreased comparing with the case that the surface is partiallyremoved without controlling the reducing force in float furnace 22. Whenthe reducing force in float furnace 22 is not controlled, as shown inFIG. 2, Sn⁺⁺ exists in a range from the glass surface to depth of about15 μm. For thoroughly removing Sn⁺⁺, the glass substrate having a largearea must be uniformly removed by thickness of 15 μm or more, preferably20 μm or more. This removing process requires mirror finish, andincreasing the removed thickness increases cost extremely, so thatdecrease of the removed thickness is extremely economical.

The surface removing process may employ a chemical method or a physicalmethod. In the chemical method, the glass substrate surface is etched bydipping glass substrate 100 into etchant 30 such as aqueous hydrofluoricacid or aqueous sodium hydroxide. The physical method includes a buffingmethod or a sand blasting method. Sufficient surface removing thicknessis about 3 to 15 μm, as shown by the study of the reflectance.

In a method shown in FIG. 9, surface removal is performed in surfaceremoving furnace 29, Sn⁺⁺ content in glass substrate 100 is analyzedagain by reflectance measuring device 32 in a second surface analyzingprocess, and surface removal is performed again if necessary. Thus, thesurface analyzing process and the surface removing process are repeatedand the surface state of the glass substrate is severely controlled,thereby further increasing the advantage of the present invention.

When the Sn⁺⁺ content in the glass substrate is determined to be higherthan the allowance, the glass substrate may be used as a back-side glasssubstrate of an image display device. When the Sn⁺⁺ content in the glasssubstrate is determined to be not higher than the allowance, the glasssubstrate may be used as a front-side glass substrate of an imagedisplay device.

When a PDP is the image display device manufactured using the glasssubstrate formed as discussed above, the PDP does not generate yellowingthat is so strong as to affect the image display characteristic, and cansufficiently display an image.

An investigation result of the PDP manufactured in accordance with thepresent invention is described.

The surface of a glass substrate (PD-200 manufactured by Asahi Glass Co.Ltd.) manufactured by the float method is partially removed so thatvarious amount of the reducing layer remains on the surface of the glasssubstrate. In other words, a maximum value of ΔR(λ)=R_(S)(λ)−R_(B)(λ),namely difference between reflection spectrum R_(S)(λ) and reflectionspectrum R_(B)(λ) in a wavelength range of 210 to 250 nm, is 0.1%, 0.8%,2.1%, 3.3%, or 4.0%. Specifically, surface removal is performed bydipping the glass substrate into etchant composed of aqueoushydrofluoric acid (10%), and the surface removing thickness iscontrolled using the dipping period. When temperature of the aqueoushydrofluoric acid is set at 27° C., etching speed is 2 μm/min. After thedipping for a predetermined period, the glass substrate is washed. Then,reflection spectrum is measured.

Using these glass substrates, three kinds of PDPs having differentresolution and structure are manufactured, and a relation betweenreflection spectrum difference ΔR(λ) and coloring degree (b*) byyellowing of the PDPs is evaluated.

PDP111 corresponds to video graphics array (VGA) (480×640 pixel), andhas a transparent electrode between an Ag electrode (bus electrode) anda glass substrate. PDP222 corresponds to extended graphics array (XGA)(768×1024 pixel), and has a transparent electrode between an Agelectrode and a glass substrate. PDP333 corresponds to XGA and has notransparent electrode between an Ag electrode and a glass substrate.

Table 1 shows a measurement result of reflection spectrum difference ΔR(λ) and coloring degree (b*) by yellowing of three kinds of PDPs. Thevalue of b* is preferably as small as possible, but, actually, theyellowing has no particular problem when b* is 2 or smaller. The PDPshave no problem as an image display device in the following conditions:

-   -   ΔR(λ) is about 3% or lower in PDP111 having a transparent        electrode between the Ag electrode and the glass substrate and a        wide pixel interval;    -   ΔR(λ) is about 2% or lower in PDP222 having a transparent        electrode between the Ag electrode and the glass substrate but a        narrow pixel interval; and

ΔR(λ) is about 1% or lower in PDP333 having no transparent electrode.TABLE 1 b * ΔR [%] PDP111 PDP222 PDP333 0.1 0.4 0.4 0.5 0.8 0.8 0.6 1.32.1 1.2 2.3 2.2 3.3 2.0 2.8 4.2 4.0 2.4 3.4 5.5

The advantage of the present invention is useful for not only a PDP butalso an image display device having the structure where an Ag electrodeis formed on a glass substrate having Sn⁺⁺ on its surface. This glasssubstrate is a glass substrate formed by the float method, for example.

INDUSTRIAL APPLICABILITY

The present invention provides an image display device that can suppressyellowing from occurring on a glass substrate manufactured by the floatmethod even when an Ag electrode is formed on the glass substrate, andhas high image display quality. The present invention provides amanufacturing method of the glass substrate for use in the image displaydevice.

1-5. (canceled)
 6. An image display device employing a glass substratein which a mean reflectance at wavelength of 200 to 250 nm is 5% orlower. 7-11. (canceled)
 12. An evaluating method of a glass substratefor an image display device, wherein Sn⁺⁺ content in the glass substrateis analyzed using a mean reflectance at wavelength of 200 to 250 nm.